The Human Immunodeficiency virus (HIV) is the causative agent of the Acquired Immunodeficiency Syndrome (AIDS). Like all retroviruses the genome of the virus encodes the Gag, Pol and Env proteins. In addition, the viral genome encodes further regulatory proteins, i.e. Tat and Rev, as well as accessory proteins, i.e. Vpr, Vpx, Vpu, Vif and Nef.
Despite public health efforts to control the spread of the AIDS epidemic the number of new infections is still increasing. The World Health Organization estimated the global epidemic at 36.1 million infected individuals at the end of the year 2000, 50% higher than what was predicted on the basis of the data a decade ago (WHO & UNAIDS. UNAIDS, 2000). Globally, the number of new HIV-1 infections in 2000 is estimated at 5.3 million.
Given the steady spread of the epidemic, there is still a need to bring an effective vaccine to the clinic. A number of different HIV-1 vaccine delivery strategies such as novel vectors or adjuvant systems have now been developed and evaluated in different pre-clinical settings as well as in clinical trials. The first vaccine candidate that entered a phase-III clinical trial is based on envelope gp 120 protein in alum (Francis et al., AIDS Res. Hum. Retroviruses 1998; 14 (Suppl 3)(5): S325-31). The phase III trials have been started although the vaccine did not prove to be too successful in the earlier phase II trial.
Following many years of prophylactic vaccine efforts based on envelope antigens, more recent efforts have focused on the use of regulatory proteins such as Tat, Nef and Rev as candidate vaccine antigens. The use of these regulatory antigens in therapeutic settings has been ongoing for several years (Miller et al., Nature Medicine 1997, 3, 389-94, Calarota et al., Lancet 1998, 351, 1320-5, Ayyavoo et al., AIDS, 2000, 14, 1-9). More recently the use of these antigens in prophylactic vaccine studies in small pre-clinical trials has revealed promise. The use of Tat and Rev, or Tat alone as a prophylactic vaccine candidate, was demonstrated to control SIVmac (Osterhaus et al., Vaccine 1999, 17, 2713-4). Moreover, there are indications that CTL directed towards the virus early regulatory proteins are important for eliminating infected cells prior to their high level production of mature virions (van Baalen et al., J. Gen. Virol 1997, 78, 1913-8; Addo et al., PNAS, 2001, 98, 1781-6).
Although the regulatory/accessory proteins of HIV induce an effective immune response, most, if not all, of them have serious side effects, which limit up to now their use as vaccine: Nef, Tat and Vpu have been shown to play a role in the down regulation of CD4+ and/or MHC class I expression (Howcroft et al., Science, 1993, 260, 1320-2; Schwartz et al., Nature Med. 1996, 2, 338-42; Swann et al., Virology, 2001, 282, 267-77; Janvier et al., J. Virol., 2001, 78, 3971-6, Weissmann et al., PNAS 1998, 95, 11601-6). It is known that Tat mediates acute immune suppression in vivo (Cohen et al., PNAS, 1999, 96, 10842-10847). Immunosuppressive effects have also been described for Vpr (Ayyavoo et al., Nature Med., 1997, 3: 1117-1123). It has been described that Vpr and Vpx have differential cytostatic and cytotoxic effects in yeast cells (Zhang et al., Virology, 1997, 230, 103-12). Thus, most, if not all accessory/regulatory proteins of HIV seem to have functional properties that are not desired in a vaccine formulation.
Attempts to reduce the harmful effects of the HIV proteins are disclosed in WO 02/06303. In particular, WO 02/06303 discloses a fusion protein including amino acid sequences of HIV Vif, Vpu and Nef, wherein the component proteins are contiguous with another component protein or separated by non-component proteins such as amino acid sequences, which make up proteolytic cleavage sites. It is disclosed that it is preferred to use those fusion proteins that comprise proteolytic cleavage sites between the component proteins. Since the component proteins are separated by proteolytic cleavage sites native HIV proteins are produced that are known to be harmful. To reduce any harmful effects of the HIV proteins that result from the cleavage of the fusion protein WO 02/06303 suggests using attenuated proteins. Thus, WO 02/06303 teaches to use a fusion protein comprising the HIV Vif, Vpr and Nef protein, wherein cleavage sites are inserted between the HIV proteins and wherein the HIV proteins are attenuated proteins. However, the disadvantage of attenuated proteins is that the amino acid sequence of the attenuated protein differs from the amino acid sequence of the native protein so that an immunization with the attenuated protein may lead to an immune response that only weakly recognizes the native protein or that even does not recognize the native protein at all.